1. Field of the Invention
Magnetic disc memories are widely employed as peripheral data storage devices in computer and real time data storage systems. In the usual situation, a plurality of discs are mounted on a single spindle and a plurality of magnetic transducing heads are suspended from arms adjacent the flat surfaces of the discs in a position allowing data to be transcribed on, i.e. written on and read from the discs. The arms are mounted on a movable carriage whose position in controlled by a voice coil actuator. By changing the drive current in the voice coil, the heads may be positioned at any desired distance from the spindle axis, allowing access to any one of the large number of data tracks on each disc. In certain disc memories now available, these tracks are concentrically located at radial increments of approximately .005 inch per track, resulting in a radial density of around 200 tracks per inch.
At such high track densities, it is of course necessary to accurately position each data head with respect to the selected track. One method adopted for positioning of these heads involves dedicating an entire disc surface to carrying pre-recorded servo tracks, the disc carrying these tracks being referred to as a servo disc. These tracks normally have the same density desired for the data tracks, and the head dedicated to reading information on the servo track is used to determined its own location with respect to the spindle axis and the servo tracks. In the preferred situation, adjacent servo tracks have distinct patterns such that when the read gap of the servo head is positioned midway between them, the signal from the servo head winding will have an equal component in its wave form from each track, anad the head may be accurately positioned by balancing or nulling these components. The data heads which transcribe data on the remaining disc surfaces are mechanically linked to the servo head and move in step with it, so that positioning the servo head adjacent a particular servo track will automatically position the data heads at the identical radius from the spindle axis, or at such other radius as is desirable. The voice coil actuator art now permits positioning of the servo head with an accuracy of a few tens of micro-inches, and similar accuracy in the positioning of the data heads.
2. Description of the Prior Art
In attempting to increase the amount of data stored by a particular disc memory configuration it is necessary to increase either the linear density of the recorded bits, i.e. increase the bits per track, or to increase the radial density of tracks. The number of tracks per radial inch is usually increased by increasing the number of servo tracks per radial inch. This however, has certain drawbacks in that either a new servo disc must be fitted to units in the field or new servo tracks written on the servo disc. With the high positional accuracy of which voice coil actuators are now capable, it has been discovered that data track density can be at least doubled without seriously affecting transcription accuracy. In so doing, it is possible to use a single servo track (viz. the balance or null location between adjacent servo tracks) to provide a reference position for two or more data tracks, with mechanical or electrical offset providing the intermediate data head positions for the data track or tracks added between those formerly present.
One aspect of this invention involves a modified servo head having a plurality of adjacent read gaps. U.S. Pat. No. 3,514,851 (Perkins, et al.) discloses a magnetic head structure having a plurality of flux paths and read gaps similar to the servo head forming part of this invention. However, the geometric relationship of the read gaps to each other is significantly different, and that disclosed in Perkins is not felt to be desirable for the application. U.S. Pat. No. 3,656,130 (Bucklin, et al.) discloses apparatus which accomplishes one result of this invention, to provide a plurality of data tracks references to a single servo track. However, Bucklin, et al. accomplishes this by providing a plurality of data heads for a single servo head. This method does not permit easy conversion of data recording systems already in existence.